Polyethylene naphthalate (PEN) has a significantly higher glass transition temperature (T.sub.g) than polyethylene terephthalate (PET), i.e., about 120.degree. C. compared to 80.degree. C., as well as a five time improvement in oxygen barrier property. PEN is thus a desirable polymer for use in thermal-resistant beverage containers (e.g., hot-fillable, refillable and/or pasteurizable containers), and for packaging oxygen-sensitive products (e.g., beer, juice). However, PEN is more expensive (both as a material and in processing costs) than PET and, therefore, the improvement in properties must be balanced against the increased expense.
One method of achieving an article that is lower in cost than PEN, but with higher thermal and barrier properties, is to provide a blend of PEN and PET. However, blending of these two polymers often results in an opaque material with incompatible phases. Efforts to produce a clear container or film from a PEN/PET blend have been ongoing for over ten years, but there is still no commercial process in widespread use for producing such articles.
One suggested method for making substantially transparent PEN/PET blends is a solid-stating process which increases the level of transesterification (copolymerization) between the two polymers. For example, WO 92/02584 (Eastman) states that transesterification occurs when the melt blended, crystallized polymer is held at a temperature below the melting point and subjected to an inert gas flow in order to raise the inherent viscosity and/or remove acetaldehyde. This transesterification is in addition to that occurring during melt blending and molding operations. However, Eastman reports that when the level of transesterification between the two polymers is very high, the crystallinity and resultant physical properties of the blend are reduced to the point where they are undesirable for making oriented containers with good mechanical properties.
Eastman teaches the addition of a phosphorus stabilizer for controlling (reducing) the amount of transesterification which occurs during solid stating. In this way, Eastman claims to limit the amount of transesterification to an amount no greater than about 20%, based on a theoretical maximum amount of transesterification being equal to 100%. For example, in Table 2 Eastman describes the transesterification and inherent viscosity of various solid-stated PEN/PET blends, where the initial inherent viscosity of the blend was on the order of 0.55 to 0.65, and the final inherent viscosity was about 0.80 to 0.85. In a control example (50--50 PEN/PET) the final inherent viscosity was acceptable (0.86) after eight hours, but the percent transesterification (25.0) was too high (above 20%). By adding 0.5 or 1.0% Ultranox 626 (a phosphite stabilizer) in the first two examples, the Eastman process provided a final inherent viscosity of 0.80 to 0.84 after eight hours, and an acceptable percent transesterification of 17.0 or 19.0 (below 20%). The other three stabilizers/metal deactivators tested in Table 2 failed to provide the final desired inherent viscosity and transesterification levels.
Although the Eastman process may be suitable for certain limited starting materials and desired transesterification levels, it cannot be expanded generally to different combinations of intrinsic viscosity, solid-stating time, and levels of transesterification. For example, of potential interest is a blend made from precursor homopolymer PEN and post-consumer PET (PC-PET). The intrinsic viscosity of PC-PET is much higher than that of virgin fibre-grade PET, so that a blend of PEN/PC-PET would require a relatively larger amount of transesterification per unit intrinsic viscosity increase (compared to a blend of PEN/virgin PET). Hence, among other disadvantages, the prior art does not provide a process that allows a desired level of both intrinsic viscosity and transesterification level.
It is possible to make substantially transparent preforms (for blow molding into containers) with a PET/PEN blend, without solid stating, but the disadvantages are such that the process is not commercially viable. First, the preform injection molding temperature (i.e., barrel temperature) and/or the equilibration time (i.e., time in the barrel) must be increased such that the resulting process is not cost-efficient or sufficiently reproducible for a commercial process. For example, in certain cases, the barrel time would be increased by a factor of four (i.e., an increase over the standard cycle of 45 seconds of up to 180 seconds); as a result, one would probably not be able to run the process on a standard injection molding machine. Furthermore, the increase in barrel time/temperature increases the acetaldehyde (AA) levels in the preform to an unacceptably high level, such that AA is likely to be extracted into the food product and produce an off taste, particularly with a product such as bottled water. Thus, this has not proven to be the desired solution.